Maleic anhydride is of significant commercial interest throughout the world. It is used alone or in combination with other acids in the manufacture of alkyl and polyester resins. It is also a versatile intermediate for chemical synthesis. Significant quantities of maleic anhydride are produced each year to satisfy these needs.
Maleic anhydride has been produced by the catalytic oxidation in the vapor phase of n-butane, butene and other C.sub.4 fractions and benzol (i.e., benzene). Typical processes for production of maleic anhydride are taught in U.S. Pat. Nos. 4,052,417; 4,065,468; 4,089,870; 4,097,501; 4,143,056; 4,244,879; 4,392,986; 4,448,978; 4,562,228; 4,567,158; 4,596,878; 4,675,420; amoung others. Products of the oxidation are maleic anhydride, water, carbon dioxide and other by-products.
In the usual vapor-phase oxidation of an appropriate hydrocarbon to prepare maleic anhydride, the effluent product stream from the oxidation unit ordinarily contains from about 1/2 to about 2.5 volume percent of maleic anhydride, from 2 to 6 volume percent water and other components such as carbon dioxide, carbon monoxide, and unconverted hydrocarbon. The major components of this product gas stream are nitrogen and oxygen.
Conventionally, the gas stream from the oxidation is first cooled to condense about one-half of the maleic anhydride and then, after separating the condensate, scrubbing the gas stream to recover the remaining maleic anhydride.
According to known methods, the maleic anhydride is obtained from the process gases in the following manner. First, the gases are cooled to a temperture of about 55.degree. to 65.degree. C. Such tempertures lies above the so-called thaw point of maleic anhydride. In the butame process approximately 40 to 45 percent of the maleic anhydride can be condensed from the process gases. The lower the condensation temperture which is chosen, the more the water contained in the gas will be bound by the condensing maleic anhydride, thus forming maleic acid. However, the formation of maleic acid is undesirable, since the maleic acid is a solid product and is poorly soluble in molten maleic anhydride. Such leads to fouling of the condenser and plugging up of the downstream apparatus, which in turn, results in the loss of throughput and yield.